You shouldn't force people to overwrite essential game files (like .vpk or .dat files). Instead, better solution is to put subtitles_english.dat in a separate folder (for example "Left 4 Dead 2\subtitles_fix\resource\subtitles_english.dat") and to only change "gameinfo.txt" in left4dead2 folder to include your new folder (subtitles_fix). This way, people can play on official servers without problems because only gameinfo.txt is changed.

Of course it works! :) One more tip: because you're changing 2 subtitles_english.txt (from DLC1 and DLC2), you can merge them into one subtitles_english.txt, which you will then compile into subtitles_english.dat. Otherwise, you'll need 2 folders for 2 .dat files, which is not so convenient.